A blast furnace wall, in particular, the hearth side wall is the portion which determines the life of the blast furnace. Therefore, prevention of damage to carbon fire-bricks composing the hearth side wall is a most important item. The causes of damage to carbon fire-bricks laid on the hearth side wall are corrosion caused by molten iron and embrittlement caused by thermal stress. In order to prevent damage to carbon fire-bricks, it is most effective to intensely cool the high heat load section on the blast furnace wall.
In respect of the method of cooling the hearth side wall of the blast furnace, there are provided two methods. One is a method of cooling the hearth side wall by circulating water in staves, and the other is a method of cooling the hearth side wall by spraying water on a shell of the blast furnace.
In this case, explanations will be made of a structure of the hearth side wall equipped with common staves for cooling. As shown in FIG. 1, carbon fire-bricks 4 are layered on the inside of the blast furnace. Between the layer of carbon fire-bricks 4 and the shell 1, there are provided stamping refractories 3, staves 5 and castable refractories 2. On a bottom hearth portion T of the blast furnace, fire-bricks 12 are layered, and cooling pipes 13 are arranged on the bottom hearth portion T. Therefore, the hearth side wall R of the blast furnace is cooled by the staves 5, and at the same time, the bottom hearth portion T is cooled by the cooling pipes 13. Reference numeral 10 is a tap hole.
As the conventional staves 5, a stave 6 made of cast iron shown in FIGS. 2A and 2B is mainly used. This stave 6 is composed in such a manner that the stave pipes 7 having cooling water passages 15 are cast at predetermined intervals. In order to prevent the occurrence of carburizing caused in the process of casting and also in order to reduce a thermal shock, the stave pipe 7 is coated with marshite 8 which functions as a heat insulating layer. In the stave pipe 7, there are provided a water feed pipe 14a for feeding cooling water and a water discharge pipe 14b for discharging cooling water.
The hearth side wall of the blast furnace is cooled when cooling water flows in the stave pipe 7 and also when heat is radiated from the shell 1. However, a quantity of heat not less than 95% of the heat to be removed from the side wall is taken away by cooling water flowing in the stave pipe 7. Accordingly, in order to enhance the cooling capacity for cooling the hearth side wall, it is effective to reduce a heat resistance between the carbon fire-bricks 4 and cooling water in the stave 6.
For this reason, improvements have been made to enhance a coefficient of thermal conductivity (inverse number of heat resistance) between the carbon fire-bricks 4 and the stamping refractories 3. Therefore, the cooling capacity for cooling the hearth has been enhanced.
However, the heat resistance of marshite 8 coated on the surface of the stave pipe 7 in the stave 6 made of cast iron is very high. Therefore, this increase in the heat resistance of the stave 6 made of cast iron has been a problem to be solved.
In order to solve the above problems, Japanese Unexamined Patent Publication (Kokai) No. 6-158131 discloses a technique in which the cooling pipe is made to come directly into contact with the stamping refractories 3 or the carbon fire-bricks 4. According to this method, the thermal resistance of the stave 6 made of cast iron can be eliminated. Therefore, the heat resistance between the carbon fire-bricks 4 and the cooling water flowing in the cooling pipe can be reduced.
However, the following problems may be encountered in the above cooling system. The above cooling system is unlike the conventional stave cooling system in which the surface of the stave 6 made of cast iron is contacted with the surfaces of the carbon fire-brick 4 via the stamping refractories. Accordingly, in the above cooling system, when the carbon fire-bricks 4 are expanded in the operation of the blast furnace, due to a difference of thermal expansion between the carbon fire-bricks 4 and the shell 1, the cooling pipe is compressed, so that the cooling pipe or the carbon fire-bricks 4 are damaged, or alternatively a gap is caused between the cooling pipe and the carbon fire-bricks 4, so that the heat resistance is increased. As a result, the reliability of the installation is deteriorated.
In other words, as compared with the time at which the blast furnace was constructed, when the blast furnace is operated for production, a gap more than several tens of mm is caused between the carbon fire-bricks 4 and the shell 1. This difference of thermal expansion is absorbed by the contraction of the stamping refractories 3 in the conventional stave cooling system. However, according to the invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-158131, no consideration is given to this point, and there is such a problem as the cooling pipe and the carbon fire-bricks 4 are damaged and as the heat resistance is increased.
Japanese Unexamined Patent Publication (Kokai) No. 55-122810 discloses a technique, which will be described as follows. The stave body is composed of a plate made of copper or copper alloy, the heat conductivity of which is good. A plurality of holes are formed by drilling in the longitudinal direction of the plate, and the end openings are closed up. After that, connecting ports for connecting the cooling water pipes are formed on the back side of the plate. The above stave cooling system is adopted for a shaft portion of the blast furnace.
When the above stave is applied to the shaft portion of the blast furnace in which the fluctuation of a heat load caused by gas in the blast furnace is directly imposed on the stave, the efficiency is high because the cooling capacity of the stave is large and further no carburizing of copper is caused by the carbon contained in the blast furnace gas.
However, on the hearth side wall of the blast furnace, it is presupposed that the carbon fire-bricks 4 must remain inside the blast furnace. Accordingly, the stave is cooled via the front carbon fire-bricks 4 and the stamping refractories 3. Due to the heat resistance of these portions, even if the coefficient of thermal conductivity of the base metal of copper is high, the overall coefficient of thermal conductivity is not so high, that is, the cost is increased too much with respect to the improvement in the cooling capacity. In the structure of the stave disclosed in the above patent publication, it is necessary to provide a cooling water feed port and a cooling water discharge port for each cooling water passage in the longitudinal direction of the plate composing the stave. Accordingly, the number of pipe attaching sections to be connected with the cooling water feed port and the cooling water discharge port is increased. Therefore, the number of openings formed on the shell 1 is greatly increased in the case of installation of the stave. Accordingly, the above stave is disadvantageous in that the shell thickness is increased and the number of gas sealing portions to seal up the openings is increased.